Most printed circuit boards employ copper as the metal which defines the conductive circuit paths of the printed circuit board. It is not infrequent after the manufacture of the printed circuit to store the printed circuit boards for substantial periods of time before further processing, such as by insertion of circuit elements and devices and soldering of these materials to the circuit board. Due to the fact that copper or copper containing metal is gradually contaminated by the atmosphere, the solderability of the copper over time decreases. In the past, it has been found necessary in order to preserve the solderability of the copper on printed circuit boards, that solder be plated onto the copper immediately or shortly after the bare board is complete. While this method has been effective in preserving the solderability of the boards, it is time consuming and costly and results in problems, e.g., shorts on fine line, high density printed circuits. Consequently, other means for preserving the solderability of copper in printed circuit boards is desired.
The use of various azoles to inhibit corrosion of copper surfaces is known in the art. For example, U.S. Pat. No. 3,295,917 teaches treating the copper with benzotriazole vapor for a time sufficient to form a thin self-sustaining protective film or coating on the copper which retards or inhibits subsequent corrosion. In U.S. Pat. No. 3,891,470 the use of imidazole and substituted imidazoles are described for inhibiting corrosion of ferrous metals such as iron and steel. Further, U.S. Pat. No. 3,933,531 discloses treating the surface of copper or copper alloys with 2-alkyl imidazoles, or an acid addition salt thereof to prevent corrosion of the copper or copper alloy. More recently, U.S. Pat. No. 4,134,959 discloses a composition for preventing corrosion of metal surfaces in contact with aqueous systems which comprise azole-phosphate dopants in the aqueous systems. The useful azoles as described therein include pyrazoles, imidazoles, isoxazoles, oxazoles, isothiazoles, thiazoles and mixtures thereof. The source of phosphate ion thereof can come from materials such as phosphoric acid, disodium phosphate, sodium tripolyphosphate, tetrapotassium pyrophosphate and the like. The combined azole-phosphate system is added to the aqueous material in contact with the metal for preventing corrosion thereof in concentrations of from 0.1 to 50,000 ppm of each of the azole and the water soluble phosphate.
It may be noted that none of the above references discuss the effect of aging of a copper surface in terms of its solderability and that several of the references deal with the use of the corrosion inhibitor as a dopant in a continuously present aqueous system as opposed to a surface treatment of the metal which is then removed from the treating system and generally stored for a period of time prior to further operations, e.g., applying solder thereto. We have now discovered an improved method for treating the surface of copper conductors so as to maintain their solderability for extended periods of times.